AASHTO TF 28:1990 Task Force 28 Report - Guidelines And Guide Specifications For Using Pozzolanic Stabilized Mixture (base Course Or Subbase) And Fly Ash For In-place Subgrade Soil Modifications Includes following subsections: Guidelines for Usage of Pozzolanic Stabilized Mixture (PSM) Base Course or Subbase; Guide Specification for Pozzolanic Stabilized Mixtures (PSM) Base Course or Subbase;